These days, mobile information terminals, video recorders, and the like are being downsized and becoming more and more portable. This has presented a problem of how to reduce the power consumption of image display devices used in those terminals and recorders.
Image display devices capable of reducing power consumption include reflective type liquid crystal display devices and transflective type liquid crystal display devices.
Reflective type liquid crystal display devices display images by utilizing outside light such as sunlight and indoor lighting and controlling the amount of light reflected by a reflective plate. This makes a back light needless and realizes a reduction in power consumption. In addition, high visibility can be secured for the display screen even under strong outside light such as direct sunlight, and therefore, the display devices are often used in instruments for mobile phones.
Transflective type liquid crystal display devices display images by utilizing light both from outside and the back light. This enables it to turn off the back light when images are displayed in bright surroundings, and thus realizes a reduction in power consumption. Under strong outside light such as direct sunlight, high visibility can be especially secured by reflecting outside light. In dark surroundings, on the other hand, visibility can be secured by turning on the back light.
Reflective plates equipped in those two types of liquid crystal display devices are preferably without specularity. This is because if light is specular-reflected, a light source appears on a reflective plate and little light is reflected except in this place of appearance, making the display screen dark. In such a case, images are displayed brightly only when seen from the regularly reflected direction of light, and images are very dark when seen from other directions. That is to say, brightness can not be secured for displayed images except in the regularly reflected direction of light, and therefore, the display screen can not be viewed naturally. In view of this, it is indispensable for reflected light to be diffused.
There are mainly two methods to diffuse reflected light. 1) To form the surface of a reflective plate into fine protrusions and depressions.
2) To provide a diffusion film for light diffusion on the viewer's side of a liquid crystal display device.
In the method 1) among these methods, since reflective properties depend on protrusions and depressions on the surface of the reflective plate, the protrusions and depressions are extremely important elements to decide display properties of a liquid crystal display device.
However, even if a liquid crystal display device has a reflective plate with protrusions and depressions, there is a problem of a dark display screen in dark surroundings with extremely small outside light. Specifically, when outside light comes into the device, the light first passes through a liquid crystal layer in a liquid crystal cell. Then, the light is reflected by the surface of the reflective plate on the backside of the liquid crystal layer, and then passes through the liquid crystal layer again to be extracted as display light. That is to say, when outside light which has come inside the device goes out of the device, the amount of the light is decreased extremely. When the amount of outside light is inherently small, almost none of it is utilized as display light, thus making the display screen dark. Especially for a liquid crystal display device having a color filter to absorb light, the amount of light decreases considerably.
In view of this, for reflective type liquid crystal display devices, it is required to further increase the efficiency of light utilization. This is preferable also for transflective type liquid crystal display devices because power consumption is reduced.
One of methods to increase the efficiency of light utilization is to control the shape of a (diffusion) reflective plate so that incident light is reflected over an effective range. In SHARP TECHNICAL JOURNAL, vol. 74 (1999), on pages 41–45, there is disclosed an ideal distribution of tilt angles of protrusions provided on a reflective plate. FIG. 1 is a graph showing the relationship between tilt angles of protrusions and the presence probability of the tilt angles. A symbol (a) in the figure shows that there is almost no flat portions (where the tilt angle is 0 degrees) on the surface of the reflective plate, and protrusions having a tilt angle of up to about 8 degrees are provided on the surface. In addition, the presence probability of the protrusions increases up to 8 degrees, and no protrusions of more than 8 degrees of tilt angle exist. In such tilt angle distribution, light is designed to be reflected uniformly over a range of about 25 degrees from the regularly reflected direction, and the light is not dispersed outside the range.
The reflective properties of a reflective plate having the tilt angle distribution can be described as follows. FIG. 2 is a graph showing the relationship between an incident angle (degree) of incident light and the obtained gain. Gain is a value obtained such that the brightness of incident light with respect to an incident angle is divided by the brightness of the incident light with respect to a (diffusion) reflective plate. In the figure, reflective properties in the case of the ideal tilt angle distribution (corresponding to the symbol (a) in FIG. 1) are shown as a solid line (a). The viewer looks at the display screen mostly from the range of 25 degrees from the normal direction. Therefore, if brightness increases within this range, bright images are displayed, as in the case of the reflective properties shown as the solid line (a).
On the other hand, a dotted line (b) shows reflective properties in the case of a symbol (b) shown in FIG. 1, where tilt angle distribution is such that the presence probability of the protrusions peaks at 15 degrees of tilt angle. With such reflective properties, incident light angled by 50 degrees, for example, can be reflected towards the normal direction. In other words, incident light angled by various degrees is reflected to have widespread and almost uniform brightness. Therefore, such reflective properties provide high light diffusion properties. However, brightness towards the normal direction is lower than the solid line (a). Note that when tilt angle is 20 degrees for example, incident light is not released out of the display device.
In addition, a symbol (c) shown in FIG. 1 is the case where the peak of the presence probability of the protrusions is in the region of small tilt angles. In this case, regular reflection components increase and the reflective plate has specularity. Accordingly, obtained reflective properties are shown as a dotted line (c) in FIG. 2. Such reflective properties cause extremely large variations in brightness with respect to incident angles.
Thus, since the reflective properties of the reflective plate depend on the tilt angle distribution of the protrusions, it is important to control the tilt angle distribution of the protrusions in order to effectively utilize incident light.
There are various methods to produce a reflective plate, among which are to roughen the surface of a reflective metal film, and to pattern a resin layer to be protrusions and depressions by etching. When a resin layer is patterned to be protrusions and depressions, the use of a thermoplastic photo-resist makes it easy to form a uniform pattern. This is specified as follows. FIGS. 3A and 3B are cross sectional views illustrating the process of forming a protrusion. First, a photo resist layer is formed on a substrate 101 and exposed through a mask having a predetermined pattern, and then developed. By these steps, protrusions 102 each having a rectangle cross section are formed. Further, by heat-treatment, corner portions of each of the protrusions 102 are heat-fused, forming protrusions 103 each having a smoothly curved surface as shown in FIG. 3B. However, if there is a wide distance between the protrusions 103, a flat portion 104 is also formed. This provides specularity to the reflective properties of the obtained reflective plate.
In Japanese Unexamined Patent Application No. 6-27481A, there is disclosed a reflective plate that solves the problem of specularity. This publication discloses a reflective plate wherein wave-shaped picture element electrodes with their upper surfaces being continuous are formed on an insulative substrate. This reflective plate is fabricated as follows. A first polymeric resin layer is formed on the insulative substrate and patterned to form protrusions. Next, a second polymeric resin layer is formed over the insulative substrate having formed thereon the protrusions in order to fill gaps between the protrusions. Thereby, the second polymeric resin layer also becomes protrusion-depression shaped. However, the tilt angles of the second layer's protrusions are smaller than those of the protrusions of the first layer. In view of this, by reducing flat portions, regularly reflected light can be reduced. However, in the above-described method, since there is an extra step of coating the second polymeric resin layer and drying it, there arises a problem of increasing fabrication costs.
In order to solve the problem of increasing fabrication costs, the present inventors attempted a method to form a reflective plate having a protrusion-depression surface by forming protrusions without gaps using a single layer. Specifically, a photo-resist was coated on a substrate and light-exposed through a mask. A pattern of the mask was such that a gap between light shielding portions was set to be 5 μm or less. After the photo-resist was developed, protrusions each having a rectangle cross section were formed. Each gaps between the protrusions was 5 μm or less. Further, the rectangle protrusions were softened and deformed by heat treatment to fill the gaps therebetween. Among the gaps, gaps of 3 μm or less were filled up and adjacent protrusions bonded to each other, thus forming a flat portion. As a result, tilt angle distribution was different between a portion in which the protrusions were distant and a portion in which the protrusions bonded, causing non-uniform reflective properties on the same plane. As a result, there arose a problem that unevenness was recognized on the display screen. In addition, when a reflective layer made of Al, Ag, or the like is formed on a source electrode and a drain electrode, there is a danger of a shortcut between the source electrode and the reflective layer at a portion without a resist.
In addition, there is another method that the step of development is stopped halfway so that a part of a resist film (hereinafter referred to as a remaining film) remains between the protrusions of the resist. When this method is employed, since the protrusions originally form a continuous surface, a continuous protrusion-depression surface is formed by melting (heat-fusion). However, the amount of deformation during the melting step varies depending on the thickness of remaining films, and the amount of deformation decreases as the remaining films become thinner. Therefore, to control the tilt angles of the protrusions, it is required to precisely control the thickness of the remaining films. However, the developing speed of the resist film depends on many factors such as temperature, the ability of a developer, and how much the resist film is fatigued. This makes it difficult to stop the development halfway and have the same amount of remaining films to control the tilt angles.
Also in Japanese Patent No. 2698218, there is disclosed a method to form the protrusion-depression surface of the reflective plate. FIG. 4A is a plan view schematically showing a reflective plate disclosed in this publication. FIG. 4B is a cross section taken along the line A—A in FIG. 4A.
As shown in FIG. 4A, a multiplicity of protrusions 112 distant from one another are provided on a substrate 111, and a reflective film 113 is provided to cover the protrusions 112. By providing the protrusions 112, the reflective plate is provided with light dispersion properties, and the appearance of a light source on the display screen is reduced. However, in this prior art example, since the protrusions 112 are made distant from one another, gap portions 114 are flat. In addition, since the area of the gap portions 114 makes up much of the reflective plate as a whole, there is a peak for reflected light in the regularly reflected direction. As a result, a light source appears on the display screen, causing a problem of making the display screen dark.
In view of this, a reflective plate that has resolved this problem is disclosed in Japanese Patent No. 2756206. FIG. 5A is a plan view schematically showing the reflective plate disclosed in this publication. FIG. 5B is a cross sectional view taken along the line B—B in FIG. 5A. According to this publication, there are protrusions and depressions. A first film 122 having protrusions is provided on a substrate 121. To cover the first film 122, a second film 123 is formed. On the second film 123 is formed a reflective film 124. In such a configuration, the formation of the second film 123 smoothes the surface of the first film 122 and flat gap portions 125, realizing a smoothly curved surface in a protrusion-depression manner. This results in removal of flat portions that were considered as a problem in the foregoing example and realizes preferable light dispersion properties without a sharp peak in the regularly reflected direction.
However, in the latter prior art example, in the formation of the reflective plate, the step to form the first film 122 having the protrusions on the substrate 121, and the step to form the second film 123 by coating liquid over the first film 122 and hardening the liquid were required. That is to say, two steps were required to form the protrusion-depression shape, which was redundant.